Self-assembly of a conjugated polymer: from molecular rods to a nanoribbonarchitecture with molecular dimensions

The growth from solution of novel end-functionalised poly(para-phenyleneeth ynylene)s (PPEs) on flat solid crystalline substrates was investigated. The macromolecular structure of PPE at the interface between thc basal plane o f graphite anti, an organic solution was visualised with molecular resoluti on by scanning tunnelling microscopy. It revealed a two-dimensional nematic -like molecular order of the polydisperse polymer, while a corresponding sh orter and monodisperse oligomer exhibited an epitaxial 2D crystal structure . In contrast, on the insulating substrate mica, PPE can self-assemble into micrometer-long nanoribbons. Their cross-section was determined quantitati vely by scanning force microscopy in tapping mode; this revealed a typical thickness of two molecular layers and a width distribution that is well des cribed by the distribution of molecular weights, according to the Schulz-Zi mm distribution. This indicates that the polymers are fully extended in the ribbons and oriented with the conjugated backbone parallel to the substrat e and perpendicular to the long axis of the ribbons. Unlike previous studie s carried out interfacing single molecules to metallic probes, we propose-h ere functionalised ribbons as polymolecular architectures, which could be u sed to interconnect gold nanoelectrodes in a molecular-scale electronic dev ice.